Alternative energy sources are being given renewed attention due to the high price of oil, environmental concerns associated with hydrocarbon-based technologies and political instability in oil producing areas of the world. One such potential source of energy is solar power. Another is thermoelectric power.
Solar energy, of course, has been the subject of study and of commercial exploitation for some time. A great deal of time and effort has been spent researching how best to convert the sun's energy into power usable in modern society. While many different techniques have been devised, photovoltaic cells are by far the most common and commercially successful. The operation of a photovoltaic cell is well understood. When the cell is exposed to solar radiation it absorbs photons of a certain energy. The photons transfer energy to electrons in the crystal lattice of the silicon. The transferred energy “excites” the electrons into the conduction band, creating the possibility of a flow of charge carriers through electrical contacts coupled to the cell. For purposes of this application, a photovoltaic cell is a device capable of converting photons from the sun into electricity by the generation of charge carriers in a light-absorbing material.
But photovoltaic cells suffer from significant limitations of efficiency. One reason for the inefficiency is the inability of low energy photons to move valence band electrons to the conduction band in the conventional materials used in modern photovoltaic cells. Another related reason is the conversion of much of the solar energy to which the cells are exposed to waste heat. And while research continues into better materials capable of converting more of the available solar energy to electricity, to date only minimal progress has been made in that direction. Thus, a need exists for methods and materials that increase the efficiency of photovoltaic cells.
Another alternative energy source that is only recently coming back into vogue is thermoelectric energy. Almost two centuries ago it was discovered that a temperature gradient in certain materials can create a voltage within the material. The voltage is caused by the diffusion of electrons from a hot region to a cold region, or vice versa. This is the so-called thermoelectric, or Seebeck, effect. During the middle part of the twentieth century the Seebeck Effect was the subject of some research. However, once it was determined that conventional materials possessed only relatively low “figures of merit”—and therefore could create only very small thermoelectric voltages—interest in the phenomenon diminished.
The figure of merit, usually designated “Z”, is a quantity describing the thermoelectric characteristics of a certain material as a function of temperature. It is the product of three quantities of a particular material—the electrical conductivity, the inverse of the thermal conductivity and the “Seebeck coefficient”—multiplied by the temperature. It is important to recognize for purposes of the present invention that the Seebeck coefficient can be either positive or negative. The sign, or polarity, of the Seebeck coefficient indicates the “direction” of the voltage created by a temperature gradient, that is, whether the hot side is positive or negative.
The prior art recognized that for conventional materials the non-temperature quantities that comprise the figure of merit for any particular material vary with each other so that it was very rare to identify a material with a figure of merit much greater than 1. Interest in using the thermoelectric effect to generate power therefore diminished. And while more recently interest in developing new materials having higher figures of merit has increased, the successful exploitation of the Seebeck effect in power generation has been very limited. What is required, therefore, is a use of thermoelectric technology that more successfully generates energy.
Combinations of photovoltaic and thermoelectric technology have been attempted in the past, with little or no success. It appears that prior attempts to combine both technologies have not created sufficient power to justify the added expense such systems required, or perhaps simply did not work. In any event, it appears that the art has lost interest in such combinations, and instead appears to be focused on the development of thermoelectric generators outright. What is needed, therefore, is a more efficient and powerful combination of solar and thermoelectric technology.